The invention relates generally to an ultrasound probe used in ultrasonic imaging of the human anatomy and, more particularly, to a technique for actively cooling the ultrasound probe.
Ultrasound imaging systems have become ubiquitous in the field of medical imaging and diagnostics. Typically, the ultrasound imaging system includes an acoustic probe (ultrasound probe) that is held against a patient. The probe includes acoustic transducers within the probe housing. Each transducer is made of piezoelectric material or electrostatic elements that transmits and receives ultrasound waves, which in turn facilitate the imaging of the internal tissues of the patient. The alternating release and absorption of acoustic energy during transmission and reception creates a thermal build-up in the probe due to acoustic losses being converted into heat.
To obtain the best performance from an ultrasound system it may be desirable to operate the acoustic probe and its associated transducers at a maximum permissible acoustic intensity, such as that allowable by the U.S. Food and Drug Administration. This will enable improvement of the quality of ultrasonic images by increasing the penetration of the acoustic waves so as to maximize the signal to noise ratio for the given system and transducer, and to ensure that imaging performance is not limited by the inability to emit the full allowable acoustic intensity. However, operating the acoustic probe and its associated transducers at higher acoustic intensities may disadvantageously result in the production of excessive heat in the transducer assembly. The amount of heat that can be allowed to build up on the exterior of an ultrasound probe must be within prescribed limits. There exist practical and regulatory limits on the maximum allowable external/surface temperature of an acoustic probe at points of contact with the patient and a technician while performing an imaging procedure. Meeting these goals depends, ultimately, upon the ability to dissipate or extract heat from the probe.
Additionally, the surface temperature of the ultrasound probe must be low enough to avoid harm to the patient and discomfort to the operator. The patient as well as the technician generally prefer to be in contact of a comfortably cool probe during imaging. Further, increased internal temperatures may affect the operational characteristics of the transducer components, thereby reducing their efficiency and/or operating capabilities. For example, CMOS integrated circuits, which may be utilized as part of the control circuitry in the probe, operate faster and more efficiently at lower temperatures.
Moreover, as will be appreciated by one skilled in the art, materials typically employed to fabricate the transducer elements are primarily selected based upon their acoustic properties, and are generally known to possess relatively low intrinsic thermal conductivity. The low thermal conductivity of transducer assemblies may result in the overheating of the probe. Further, most of the heat generated by operation of the probe tends to build up immediately around the transducer elements, which are necessarily situated in the probe very close to the body of the patient being examined. Additionally, the transducer elements are generally isolated from one another by dicing kerfs that provide additional thermal insulation of the transducer elements. Hence, the heat generated within the transducer elements is trapped in the acoustic stack causing the face temperature of the probe to rise above the ambient temperature. It is generally advantageous to dissipate the heat that may be trapped in the array of transducer elements in order to circumvent the overheating of the contact surfaces of the ultrasound probe.
Conventionally, thermal management in ultrasound probes is accomplished with relatively simple devices such as heat conductors, which are buried in the transducer structure so that they transfer heat from the source into the body of the probe structure as quickly as possible. For example, the interior volume of the probe housing surrounding the transducer array may be filled with thermally conductive potting material, e.g., heat-conductive ceramic granules embedded in epoxy. The potting material stabilizes the construction and assists in dissipating heat, generated during pulsation of the transducer element array, away from the probe surface/transducer face toward the interior/rear of the probe. In this way heat is conducted from the critical front surface of the probe into the handle where the increased mass helps dissipate the heat evenly via natural convection.
Because the amount of electronics in conventional ultrasound probes has typically been small enough, natural convection has been sufficient to keep the probe temperature within the regulatory limits. To avoid overheating of the probe, it is common practice to include a thermistor or other temperature sensing device in the probe near the patient contact surface so as to reduce or terminate electrical power and excitations to the probe in the event of overheating.
However, ultrasonic transducer technology is rapidly evolving towards probes with higher element counts. This in turn requires more cabling and lighter-weight materials, and challenges the manufacturability of the interconnect between the individual elements and the ultrasonic imaging system. Added to this strain on the packaging technology is the availability of high levels of circuit integration in semiconductors. Because of the electrical impedance mismatch between the small elements in the transducer and the sensing electronics in the system, various means have been developed to provide active electronics within the probe handle. As electronic technology advances, it is expected that more active circuitry will be placed as near to the source of the detected signal as possible.
The application of semiconductor technology to the diagnostic ultrasonic transducer has created a new dimension in the design and fabrication of these devices. Whereas these products have traditionally been composed of passive electronic circuits and sensors of piezo-electric ceramic, the transducer is now host to active preamplifiers, transmitters, lasers, and ultimately, A/D converters and perhaps digital signal processors. This has significantly increased the requirements for operating power in the probe. This increase in operating power has necessarily led to an increase in operating temperatures. The addition of this technology into the traditionally “hand-held” ultrasonic probe creates severe strains on the ability of the mechanical designer to dispose of the heat generated by the active devices, thereby exacerbating the difficulty of thermal management within the probe. In order to make the highest quality images, the power output of the probe is managed close to the regulatory limit, creating a need to manage the thermal output of the probe.
Thus, with the advent of active devices, the above-described use of heat conductors is no longer sufficient to handle the heat load within the transducer. Ultrasound probes with more electronics in the handle require dissipating higher amounts of heat, such that cooling beyond natural convection is required to meet the regulatory temperature requirements. For example, the heat load dissipated by the simple devices available today is approximately 1 Watt. If preamplifiers are introduced into the system, which dissipate 3 milli Watt in a quiescent mode, the heat load will be increased by 9 Watts for a 3000-element probe, making a total of 10 Watts. Because the current designs are sometimes limited by the temperature of the patient contact area, there is little margin to accommodate this type of thermal output increase. Thus, there is a need to provide thermal transfer mechanisms capable of dissipating greater amounts of heat.
Proposed techniques to enhance the thermal management of the ultrasound probe typically include self-contained cooling systems such as a closed loop circulating cooling system, a thermoelectric cooler, an evaporator/condenser system, channels for circulating cooling liquid about an ultrasonic transducer structure and so forth. These techniques generally have been successful at sufficiently reducing face temperature of the probe. However, this often comes at the expense of the acoustic performance of the transducer assembly. For example, vibrations from pumped cooling fluid may degrade the quality of the image. Similarly, pressure variations during operations may damage the pump/tube. Further, leakage of the cooling fluid from the pump may adversely reduce the life of the cooling systems. Given that it is desirable to be able to operate at the maximum allowable acoustic intensity and also desirable to control the internal transducer operating temperatures as well as the surface temperature distribution of the patient and user-contacting portions of the probe's surfaces, thermal engineering is a serious consideration during transducer design.
It is therefore desirable to provide an efficient and cost effective technique for actively cooling the ultrasound probe so as to facilitate high quality diagnostic imaging by operating the probe at a higher transmit power while maintaining the surface temperature of the probe within regulatory limits. It is also desirable to reduce vibrations, pressure variations and leakage of the cooling fluid from the pump to improve image quality and life of the cooling system.